ABSTRACT Phase contrast microscopy is one of the most widely used techniques for biological imaging because it provides exquisite high-resolution images of sample morphology, without the use of sample labeling. However standard phase contrast techniques, like differential interference contrast (DIC), only work in the transmission direction and thus cannot be used when imaging thick samples. For this reason, standard phase contrast techniques have not made a great impact in in-vivo biomedical or clinical imaging. We have developed a new phase contrast technique, called Oblique Back-illumination Microscopy (OBM), that works in a reflected light configuration, and is thus amenable to in-vivo endomicroscopy applications. OBM requires no labeling and provides high resolution DIC-like images of sub-surface sample morphology. As far as we know, DIC-like imaging in an epi-detection configuration has never been demonstrated before. Our goal in this project is to demonstrate that OBM can enable clinicians to perform histopathology and tissue diagnosis in situ, without the need for a physical biopsy. We will limit our study to the evaluation of OBM endomicroscopy for preclinical cancer diagnosis. In particular, we will concentrate on colorectal imaging in rodents, in-vivo and ex-vivo. Small animal imaging with endomicroscopes presents technical challenges related to probe miniaturization that are more restrictive than human imaging. Nevertheless, we believe such imaging is a necessary first step that must be taken prior to clinical evaluations. Accordingly, our specific aims are to 1) construct a miniaturized OBM endomicroscope probe suitable for in-vivo colorectal imaging in rats, 2) perform longitudinal in-vivo imaging of a chemical rat cancer model, and 3) perform ex-vivo imaging of a genetic mouse cancer model to compare with standard H&E histopathology. A difficulty with the development of a new technique is that, while we have already established that OBM produces high resolution images in thick tissue, we do not yet have a roadmap for what role these images can play in clinical applications (precisely because no-one has been able to perform such imaging before). Our goal for this project is to establish this role by correlating OBM imaging results with the known chronology of cancer in well characterized animal models, thus enabling us to define classification criteria associated with this new technology. Ultimately, we hope this preclinical study will lead toward the development of a simple, safe, versatile, low-cost endomicroscopy technique that can be operated in conjunction with standard endoscopy for in-situ colorectal cancer diagnosis in the clinic.